Increasing main magnetic field strength in MRI improves image quality by increased polarization of the magnetic spins, which results in improved signal-to-noise ratio (SNR) and/or spatial resolution. Simultaneous multislice (SMS) acquisition is a promising technique for reducing acquisition time by exciting multiple slices simultaneously and resolving them during reconstruction thus allowing for reduced scan time or increased slice coverage.
One of the main challenges with simultaneous multislice imaging involves the peak RF power that often exceeds amplifier limits due to summation of subpulses from multiple slices. There are several strategies for addressing this peak power problem including optimizing the phases of the individual subpulses, shifting the pulses in time before summing so the peaks don't overlap, and “root-flipping.” Recently, this problem was tackled in the framework of an optimal control problem by minimizing a cost function that includes a term for excitation accuracy and another term penalizing the pulse power.
By integrating forwards and backwards in time through use of a Bloch equation simulator to calculate this cost function and its derivatives, a minimum could be found by optimizing the RF and gradient waveforms. This method was shown to achieve better multiband factors than prior approaches.
At higher field strengths B1+ inhomogeneity and local SAR concerns become increasingly problematic and therefore parallel transmission is used where several transmitters with independently controllable amplitude and phase are used to achieve flip angle homogeneity and mitigate local SAR hotspots. For slice selective excitation, a spokes excitation k-space trajectory is used where multiple spokes are applied along the kz direction for different locations in the kx-ky plane. The waveform shape will control the slice profile while the choice of kx, ky values for the spokes locations can be used to optimize the flip angle homogeneity in addition to the choice of amplitude and phase on each transmit channel. In addition to reducing flip angle inhomogeneity, the amplitudes and phases of the transmit channels should be chosen to suppress local SAR hotspots.
The IMPULSE method has been shown to be an effective way to optimize parallel transmit pulses for sequential multislice excitation in a SAR aware manner by using a distributed optimization algorithm (alternating direction method of multipliers, ADMM) to decompose the problem in such a way that allows the subproblems to be solved efficiently without the need for compressing local SAR matrices which is a drawback of most other pTx design algorithms.
Finally, if the peak power still violates the peak power limit even with these optimized waveforms, a time-optimal VERSE algorithm is used to enforce the peak power limit for the pulse with minimum duration.
What is needed is a method for designing pTx-SMS pulses to achieve uniform flip angle maps and mitigate local SAR hotspots while exciting multiple slices simultaneously.